Charging devices play an important role in electrostatographic imaging. Prior to each exposure, a photoreceptor is charged to an essentially uniform high potential (around 1000 V) by a charging device. The charge potential can be negative or positive, depending on the type of photoreceptor used. In one widely used technique for charging the photoreceptor surface, a charged-particle depositing corona wire is activated near the surface, raising the potential of the photoreceptor by a prescribed voltage (around 1000 V). Two types of chargers employ the corona wire technique, namely the Corotron and the Scotorton. The Corotron uses the corona wire directly by simply mounting a tensioned corona wire charged to a high voltage close to the photoreceptor, while the Scotorton is a Corotron that also employs an intermediate biased grid which controls the charging level. Many versions of Corotrons and Scorotrons are used in practice; one example being a Double Scorotron, which employs two parallel corona wires in charging the photoreceptor.
All of the above mentioned charging devices employing a corona wire have a common problem of corona wire contamination resulting from oxidation and contaminant accumulation; this causes irregularities in the charging process and causes an inhomogeneous potential on the photoreceptor. Consequently, corona wires or the entire corona structure must, occasionally, be replaced manually or cleaned. Manually replacing the wire has two drawbacks, first, it adds an extra maintenance burden, and second, there is a noticeable decline in image quality over time, after each replacement of wire.
A great variety of patents are concerned with the problems associated with corona wire replacement and corona wire cleaning. The Patents range from automatic wire-cleaning devices to instruments which aid a substantially manual wire replacement.
U.S. Pat. No. 5,023,748 describes a motor-driven corona wire cleaning device. A driving-wire drives a cleaning member along the corona wire and removes foreign particles which have previously accumulated on the wire. While the device is useful in removing some of the contaminant accumulation off the corona wire thus extending its working life span, it still does not obviate the need to frequently replace the corona wire.
U.S. Pat. No. 5,074,484 describes a Corotron rewiring tool, which is not part of the Corotron, for aiding manual replacement of corona wire. The tool consists of a spool of fresh corona wire from which the wire is manually drawn under low tension, and a member which locks the wire at a fixed position during manual wire tensioning in order to prevent the undrawn wire from being tensioned. A technician may use this tool for replacing corona wire, but it is still the technician who actually performs the replacement.
U.S. Pat. No. 5,008,538 describes a corona charging apparatus including properly prelocated and pretensioned corona wire. Whenever necessary, the assembled apparatus is replaced by a simple procedure with a new assembled apparatus.
U.S Pat. No. 4,746,796 describes a means for supplying a corona charger with corona wire of variable length, in accordance with the width of the processing material used by a photocopier. The invention includes a spring-loaded reel in a cassette for dispensing the corona wire, and means for pulling out the end of the wire to a desired working position. This Patent further provides means for cleaning the wire as it moves in and out of the cassette. The used wire is not replaced by the mechanism of this Patent, rather it is reused until the entire cassette is replaced when the wire is exhausted.